Cytology

Ecological Factors.

The most superficial observation shows that species are not uniformly distributed throughout their range and that certain conditions favour one kind of plant rather than another. In the ecological concept the idea of competition is never absent and the effect of the external conditions upon the constituents of the vegetation of any region is largely dependent upon their selective action. The sum total of the external circum stances surrounding the plant is termed the habitat and the va rious conditions that together determine distribution are termed the habitat factors, of which some are physical and others biological.

Climatic Factors.

Of the physical factors the most important are climatic and of these temperature would appear to play the major role. Hence we find that the main vegetation zones on the earth's surface have a latitudinal distribution and that the species which has an extensive north and south range has commonly a wide range in the east and west direction also. Whilst some species, as already noted, are almost cosmopolitan, it is true of the majority that they do not occur both in tropical and arctic regions. In some cases it has been shown that the species cam prises several geographical races with different capacities for en durance of climatic extremes, but there is little evidence to show that species can become adapted to climatic extremes that they could not initially endure. Many garden plants are cultivated in the open beyond the limits of their natural range, showing that the climatic conditions need not be lethal in order to delimit the area of a species.

By diminishing the rate of growth or inhibiting reproduction temperature may effectually prevent the maintenance of a species in competition with other plants although there is a considerable margin before the lethal limit is reached. Extremes of tempera ture, however, cause death in many plants by coagulation of the living cell contents, and in general the susceptibility to such extremes is directly proportional to the amount of water which the plant or part of the plant contains. Hence seeds which have a very low content of water are exceptionally resistent whilst suc culent shoots are especially susceptible. Rapid changes of tem perature are far more liable to prove fatal than changes of the same amplitude which occur slowly.

In tropical and sub-tropical regions where there is no dry season the vegetation consists of broad-leaved evergreen forests, but where, as in temperate latitudes, a warm and cold season alter nate, the characteristic type is deciduous woodland; whilst in still higher latitudes, where the growing season, or frostless period, is of short duration, the prevailing types of forest consist of narrow leaved evergreen conifers. A similar zonation to that presented from the Equator to the Poles is seen in the altitudinal zonation on the higher mountain masses. Such is well illustrated in Corsica where the olive characterizes the belt from sea-level to 400 metres, the chestnut from doom. to i,000m., pines (P. Pinaster and P. Laricio) and beech from i,000m. to 1,600m.; the alder (Alma suaveolens) the sub-alpine zone from 1,600m. to 1,9oom. and scrub of the dwarf juniper (Juniperus nanus) with Berberis aet nensis up to 2,000 metres.

The importance of temperature and the length of the growing season is further shown by the altitudinal distribution of indi vidual species, which varies markedly in correspondence with the latitude. For example, Polygonum viviparum attains to 1,23om. in Scotland at 56.48 N. Lat., on the Swiss alps it is found at over 2,850m. (9,000ft.), and on Mt. Everest (27.59 N. Lat.) reaches an elevation of 4,46om. (14,5ooft.). Study of comparative altitudes in different latitudes shows that the upper limits tend to rise on the higher mountains and in areas of large mountain masses, which indicates the importance of exposure conditions.

The extreme conditions tolerated by living vegetable organisms is shown by the fact that spores of bacteria can endure immersion in liquid air whilst certain blue-green algae (Phormidium lami nosum, Mastigocladus laminosus) occur in water at temperatures above 49°C. Even amongst the flowering plants the resistance of some species to cold is considerable, Larix siberica for instance growing where temperatures of — 70°C are not unknown, whilst seeds are particularly resistant to both heat and cold. Here, too, attention may be called to the extreme altitudes attained by some flowering plants, Arenaria muscosa being found at 2o,400ft. on Mt. Everest, the highest recorded station for a member of this class.